1. Field of the Invention
This invention relates to an active filter, and more particularly to an active filter formed in an integrated circuit.
2. Description of the Related Art
When forming filters in an integrated circuit, active filters which utilize feedback technique are often formed because of difficulty in constructing an inductance in the integrated circuit. A basic block of an active filter has a gain block (hereinafter called a voltage-controlled current source) which amplifies according to an incremental transfer conductance, an integrating capacitor which is a in determining the coefficient of the filter, and an impedance transfer circuit (buffer) comprising for example resistors.
FIGS. 1 and 2 show basic blocks of filters formed in integrated circuits. FIG. 1 shows a circuit including a differential circuit-type voltage-controlled current source 10 and an integrating capacitor 12. FIG. 2 shows a circuit including a feedback loop formed by a buffer amplifier 14 added to the circuit shown in FIG. 1. In FIG. 1, if the voltages at the non-inverting input and the inverting input of the voltage-controlled current source 10 equal V.sub.1 and V.sub.2, respectively, and the voltage at the capacitor 12 equals V.sub.3, the output voltage Y of the circuit in FIG. 1 is as follows. ##EQU1## wherein gm is an incremental transfer conductance of the voltage-controlled current source 10, C is a capacitance of the capacitor 12 and S is an angular frequency.
In the equation (1), gm/C determines the coefficient of the filter. If a desired coefficient of the filter is A (a constant value), the capacitance C needed in the integrated circuit is as follows. ##EQU2## It is understood that filters with different characteristics may be formed in the intergrated circuit if the capacitance C is adequately changed.
The capacitance C is controlled by the area used in forming the capacitor in the integrated circuit. In general, the capacitor occupies a larger area than other devices. The value of the capacitance realized in the integrated circuit has an upper limit because the area occupied by the capacitor becomes large as the capacitance increases. Thus, the filter coefficient realized in the integrated circuit also has a limit.